Live human bone adjacent to implants undergo simultaneous bone resorption and new bone formation. In an analogous manner, soft tissue around implants becomes remodelled after the initial wound healing period.
In bone, the above process results in adaptation of bone structure to functional demands and loading profile on the bone. At a normal bone fracture site, the dynamic interactions between bone resorption and formation ensure a healthy regeneration of the bone. In certain pathological conditions, such as loosening of orthopaedic joint prostheses, malignancy, and osteoporosis, bone resorption exceeds bone deposition. However, resorption may occur at an accelerated rate when orthopaedic implants are in contact with bone. This leads to both impaired early fixation and integration of the implant and early or late net bone loss at the site of implantation.
In soft tissue, a prolonged inflammation is more often observed around implants than around sham sites. This may be manifested as a delayed presence of inflammatory cells, such as macrophages and monocytes, as observed by e.g. immunohistochemical methods. The sustained inflammatory process is believed to result in a fibrous encapsulation of soft tissue implants.
Hydroxyapatite (HA) and tricalcium phosphate (TCP) coatings are widely used to improve the fixation of implants to bone for periods of a few weeks up to several months. However, HA and TCP coatings are expensive to prepare and diverging biological behavior has been reported. Also, HA and TCM coatings are often inadequate for long-term use. For certain applications, the currently available coated implants are not adequate and sometimes fail e.g. due to cracking after years of use.
The bone repair time around metallic implants may be in the range of months and is often described in terms of bone in-growth into threads, pores, holes and asperities. For certain applications, the implant can or should not be allowed to carry load until this process has lead to a sufficient surface/tissue binding strength. Such unloading is not always possible, and it is desirable to shorten the unloading time period.
Furthermore, some implants create a possibility for bacteria from the adjacent skin or mucous membrane to enter the interface between the implant and the surrounding tissue. In the case of so called external fixation devices, used in fracture treatment, this often leads to bone resorption and the device loosens within months. As already mentioned, conventional implants also have a tendency to cause prolonged inflammation and other undesirable tissue reactions especially in soft tissues.